1. Field of the Invention
This invention relates to a ferroelectric memory device adapted to reduce a lasting effect or imprint effect of ferroelectric memory cells. The present invention is also directed to a method of reducing a lasting effect or imprint effect of ferroelectric memory cells.
2. Description of the Prior Art
Ferroelectric memories using ferroelectric capacitors are known as nonvolatile semiconductor memories. Generally, the semiconductor memory including the ferroelectric memory is constituted with a plurality of memory cells arranged in rows and columns, with each memory cell constituting a single memory unit (a bit).
FIG. 9 shows an example of circuit constitution in the vicinity of a memory cell 2 constituting a conventional ferroelectric memory. This memory cell 2 is of the so-called "2-transistor, 2-capacitor type." The memory cell 2 is provided with a pair of cells 4 and 6. The cell 4 is provided with a ferroelectric capacitor 8. The cell 6 is constituted similarly to the cell 4 and provided with a ferroelectric capacitor 10.
FIG. 8 generally shows a hysteresis curve representing the relationship between the voltage and polarized state (electric charge) for the ferroelectric capacitor. The relationship between the voltage (the potential of the bit line BL or /BL relative to the reference voltage of the plate line PL shown in FIG. 9) and the polarized state (shown in the figure with the "electric charge" which is equivalent to the "polarized state") for the ferroelectric capacitor 8 or 10 can be represented using FIG. 8.
In FIG. 8, the state with a residual polarization Z1 is assumed as the polarized state P1, and the state with a residual polarization Z2 is assumed as the polarized state P2. The ferroelectric capacitors 8 and 10 are constituted so that they are in the polarized state P1 or P2 when no potential difference exists between their both ends.
For example, when the ferroelectric capacitor 8 is in the polarized state P1, the ferroelectric capacitor 10 is in the polarized state P2 (The state is assumed to correspond to a stored data "H."). On the other hand, when the ferroelectric capacitor 8 is in the polarized state P2, the ferroelectric capacitor 10 is in the polarized state P1 (This state is assumed to correspond to a stored data "L.").
When the polarized states of the ferroelectric capacitors 8 and 10 are known, the data stored in the memory cell 2 can be known (read).
In order to know the polarized states of the ferroelectric capacitors 8 and 10, voltages Vf1 and Vf2 produced between both ends of the ferroelectric capacitors 8 and 10 may be measured after discharging a loading capacitor (parasitic capacitances of the bit lines BL and /BL) 12, followed by bringing the bit lines BL and /BL to floating state, and applying a reading voltage Vp to the plate line PL.
According to a graphical solution using FIG. 8, when the capacitance of the loading capacitor 12 is represented with the slope of the straight line L1, if the ferroelectric capacitor 8 is in the first polarized state P1, the voltage Vf1 produced between both ends of the ferroelectric capacitors 8 becomes V1. On the other hand, if the ferroelectric capacitor 10 is in the second polarized state P2, the voltage Vf2 becomes V2. A sensing amplifier 14 is used to know the polarized states the ferroelectric capacitors 8 and 10 are in by measuring which of the voltages Vf1 and Vf2 is greater at the time of reading.
In this way, stored data ("H" or "L") corresponding to the polarized states of the ferroelectric capacitors 8 and 10 can be read by checking the polarized states.
However, a problem associated with the conventional ferroelectric memory described above is that the ferroelectric has the property of producing distortion (lasting effect or imprint effect) on its hysteresis curve when the ferroelectric is held in the polarized state for an extended period of time.
Therefore, when a data is left stored for a long period of time, the lasting effect or imprint effect occurs in the ferroelectric capacitors 8 and 10 which constitute the memory cell 2. When such an effect occurs, the voltage values Vf1 and Vf2 produced between both ends of the ferroelectric capacitors 8 and 10 vary. In particular, when a data is stored which is opposite to the data stored when the imprint effect occurred, it is difficult to correctly read the opposite data. That is to say, the function of the memory device deteriorates with time.